This invention relates to materials for use in electrophoretic media and displays. More specifically, in one aspect this invention relates to an electrophoretic medium and display containing a fluid which reduces haze. In another aspect, this invention relates to an electrophoretic medium and display with improved image stability.
Electrophoretic displays have been the subject of intense research and development for a number of years. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. (In practice, some electrophoretic displays, including some of the displays of the present invention, are capable of achieving multiple gray states, and, are stable not only in their extreme black and white optical states, but also in their intermediate gray states. Although such displays should properly be described as “multi-stable” rather than “bistable”, the latter term may be used herein for convenience.) The optical property which is changed by application of an electric field is typically color perceptible to the human eye, but may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see, for example, Kitamura, T., et al., “Electrical toner movement for electronic paper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., “Toner display using insulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Patent Publication No. 2005/0001810; European Patent Applications 1,462,847; 1,482,354; 1,484,635; 1,500,971; 1,501,194; 1,536,271; 1,542,067; 1,577,702; 1,577,703; and 1,598,694; and International Applications WO 2004/090626; WO 2004/079442; and WO 2004/001498. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media, when the media are used in an orientation which permits such settling, for example in a sign where the medium is disposed in a vertical plane. Indeed, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based ones, since the lower viscosity of gaseous suspending fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspending medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545; 6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,704,133; 6,710,540; 6,721,083; 6,724,519; 6,727,881; 6,738,050; 6,750,473; 6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068; 6,825,829; 6,825,970; 6,831,769; 6,839,158; 6,842,167; 6,842,279; 6,842,657; 6,864,875; 6,865,010; 6,866,760; 6,870,661; 6,900,851; 6,922,276; 6,950,200; 6,958,848; 6,967,640; 6,982,178; 6,987,603; 6,995,550; 7,002,728; 7,012,600; 7,012,735; 7,023,430; 7,030,412; 7,030,854; 7,034,783; 7,038,655; 7,061,663; 7,071,913; 7,075,502; 7,075,703; 7,079,305; 7,106,296; 7,109,968; 7,110,163; 7,110,164; 7,116,318; 7,116,466; 7,119,759; 7,119,772; and 7,148,128; and U.S. Patent Applications Publication Nos. 2002/0060321; 2002/0090980; 2002/0180687; 2003/0011560; 2003/0102858; 2003/0151702; 2003/0222315; 2004/0014265; 2004/0075634; 2004/0094422; 2004/0105036; 2004/0112750; 2004/0119681; 2004/0136048; 2004/0155857; 2004/0180476; 2004/0190114; 2004/0196215; 2004/0226820; 2004/0239614; 2004/0257635; 2004/0263947; 2005/0000813; 2005/0007336; 2005/0012980; 2005/0017944; 2005/0018273; 2005/0024353; 2005/0062714; 2005/0067656; 2005/0078099; 2005/0099672; 2005/0122284; 2005/0122306; 2005/0122563; 2005/0122565; 2005/0134554; 2005/0146774; 2005/0151709; 2005/0152018; 2005/0152022; 2005/0156340; 2005/0168799; 2005/0179642; 2005/0190137; 2005/0212747; 2005/0213191; 2005/0219184; 2005/0253777; 2005/0270261; 2005/0280626; 2006/0007527; 2006/0024437; 2006/0038772; 2006/0139308; 2006/0139310; 2006/0139311; 2006/0176267; 2006/0181492; 2006/0181504; 2006/0194619; 2006/0197736; 2006/0197737; 2006/0197738; 2006/0198014; 2006/0202949; and 2006/0209388; and International Applications Publication Nos. WO 00/38000; WO 00/36560; WO 00/67110; and WO 01/07961; and European Patents Nos. 1,099,207 B1; and 1,145,072 B1.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called “polymer-dispersed electrophoretic display” in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, WO 01/02899, at page 10, lines 6-19. See also copending application Ser. No. 09/683,903, filed Feb. 28, 2002 (now U.S. Pat. No. 6,866,760).
A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the suspending fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Application Publication No. WO 02/01281, and published US Application No. 2002/0075556, both assigned to Sipix Imaging, Inc.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in shutter mode.
An encapsulated, electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
Although as already mentioned, electrophoretic displays exhibit bistability, this bistability is not unlimited, and images on the display slowly fade with time, so that if an image is to be maintained for extended periods, the image must be refreshed periodically (and the intervals at which such refreshing is necessary is a convenient quantitative measure of image stability). Also, in many systems which lack image stability, it is necessary to apply so-called “blanking pulses” at regular intervals; such blanking pulses involve first driving all the pixels of the display to one optical state (for example, a white state), then driving all the pixels to the opposite optical state (for example, black), and then writing the desired image. Since such refreshing and blanking of the display inherently consumes energy, and the blanking pulses are distracting for a user who is trying to concentrate on an image, it is desirable to minimize such refreshing and blanking, that is to say, to increase the bistability of the display so that the intervals between refreshing and/or blanking of the image can be increased, and the power consumption of the display thus reduced. For example, one potential application of electrophoretic displays is in personal digital assistants (PDA's) where the inherent size and weight limitations of the device, and the need for the screen to be visible under a wide variety of lighting conditions, render low power consumption and reflective mode of electrophoretic displays very attractive. It is common for the user of a PDA to keep a single image, such as a list of telephone numbers, on the screen for an extended period, and in order to keep the energy consumption of the PDA and distractions to the user to an absolute minimum, it is desirable that this be achievable without the need for any refreshing or blanking of the display during this extended period.
It has been found, by one of the present inventors and his co-workers, that the main factor limiting image stability in electrophoretic displays of the types described in the aforementioned patents and applications is settling of the electrophoretic pigment particles under gravity. Since the rate of such settling is to a first approximation inversely proportional to the viscosity of the liquid phase in which the pigment particles are suspended, the stability of the image can be increased by increasing the viscosity of the liquid phase. Unfortunately, as is well known to those skilled in the technology of electrophoretic displays, the electrophoretic mobility of the pigment particles (the rate at which the particles move through the liquid phase under a given electric field) is also inversely proportional to the viscosity of the liquid phase, and thus the switching time of the display (the time required to change a given pixel of the display from one of its optical states to the other essentially the time necessary for the pigment particles to move through the thickness of the liquid medium) is directly proportional to the viscosity of the liquid medium. Accordingly, although it is well within the level of skill in the art to vary the viscosity of the liquid medium over a wide range, it has hitherto appeared that any chosen viscosity necessarily represents a compromise, in that increasing the viscosity will increase image stability at the cost of increased switching time. Furthermore, especially in the case of small displays such as PDA displays where it may be necessary to move through several “pages” or screens of information to find a desired datum, users will not tolerate switching times substantially in excess of about 1 second. (Although it is possible to counteract an increase in switching time due to increased viscosity of the suspending fluid by increasing the drive voltage applied to the display, this brings its own set of problems. Increasing the drive voltage necessarily increases the energy consumption of each driving pulse, and may increase the complexity and cost of the electronic circuitry required to control the driving pulses. Furthermore, in many battery-driven devices, it is not practicable to increase the driving voltage above certain limits.) Thus, it has hitherto appeared that the maximum image stability which can be achieved in a PDA or similar electrophoretic display is limited to a value which is substantially lower than is desirable for energy conservation purposes.
It is shown in the aforementioned copending application Ser. No. 10/063,236 that the addition of certain polymers to the suspending fluid used in electrophoretic displays provides an increase in image stability greater than can be accounted for by the increase in viscosity of the fluid caused by the addition of the polymer. Accordingly, the use of these polymers in the suspending fluid allows for substantial increases in image stability without excessive increase in the switching time of the display. More specifically, this copending application describes an electrophoretic medium comprising a plurality of particles suspended in a suspending fluid, the particles being capable of moving through the fluid upon application of an electric field to the medium, the fluid having dissolved or dispersed therein a polymer having a number average molecular weight in excess of about 20,000, the polymer being essentially non-absorbing on the particles. This copending application also describes an electrophoretic medium comprising a plurality of particles suspended in a hydrocarbon suspending fluid, the particles being capable of moving through the fluid upon application of an electric field to the medium, the fluid having dissolved or dispersed therein a polyisobutylene having a viscosity average molecular weight in the range of about 400,000 to 1,200,000 g/mole, the polyisobutylene comprising from about 0.25 to about 2.5 per cent by weight of the suspending fluid. Furthermore, this copending application also describes an electrophoretic medium comprising a plurality of particles suspended in a suspending fluid, the particles being capable of moving through the fluid upon application of an electric field to the medium, the fluid having dissolved or dispersed therein a polymer having an intrinsic viscosity of [η] in the suspending fluid and being substantially free from ionic or ionizable groups in the suspending fluid, the polymer being present in the suspending fluid in a concentration of from about 0.5 [η]−1 to about 2.0 [η]−1.
The polyisobutylene used in the worked Examples of the aforementioned copending application Ser. No. 10/063,236 gives good results in many electrophoretic media. However, there are certain electrophoretic media where use of polyisobutylene is not practicable. For example, polyisobutylene tends to degrade under very high shear conditions, so that if the process used to produce the electrophoretic medium requires the use of very high shear conditions, polyisobutylene may not be useful. Furthermore, as described in more detail below, there are certain fluids useful in electrophoretic media in which polyisobutylene is not sufficiently soluble to be useful in increasing image stability. Accordingly, one aspect of the present invention relates to electrophoretic media generally similar to those described in the aforementioned copending application Ser. No. 10/063,236 and having high image stability, but containing polymers other than polyisobutylene.
Another aspect of the present invention relates to reducing haze in variable transmission windows and similar light modulators. As described in the aforementioned copending application Ser. No. 10/907,140, one potentially important application of electrophoretic media is in light modulators, that is to say to variable transmission windows, mirrors and similar devices designed to modulate the amount of light or other electro-magnetic radiation passing therethrough; for convenience, the term “light” will normally be used herein, but this term should be understood in a broad sense to include electro-magnetic radiation at non-visible wavelengths. For example, the present invention may be applied to provide windows which can modulate infra-red radiation for controlling temperatures within buildings.
One potentially important market for electrophoretic media is windows with variable light transmission. As the energy performance of buildings and vehicles becomes increasingly important, electrophoretic media could be used as coatings on windows to enable the proportion of incident radiation transmitted through the windows to be electronically controlled by varying the optical state of the electrophoretic media. Effective implementation of such “variable-transmissivity” (“VT”) technology in buildings is expected to provide (1) reduction of unwanted heating effects during hot weather, thus reducing the amount of energy needed for cooling, the size of air conditioning plants, and peak electricity demand; (2) increased use of natural daylight, thus reducing energy used for lighting and peak electricity demand; and (3) increased occupant comfort by increasing both thermal and visual comfort. Even greater benefits would be expected to accrue in an automobile, where the ratio of glazed surface to enclosed volume is significantly larger than in a typical building. Specifically, effective implementation of VT technology in automobiles is expected to provide not only the aforementioned benefits but also (1) increased motoring safety, (2) reduced glare, (3) enhanced mirror performance (by using an electro-optic coating on the mirror), and (4) increased ability to use heads-up displays. Other potential applications include of VT technology include privacy glass and glare-guards in electronic devices.
The aforementioned copending application Ser. No. 10/907,140 describes various factors which are important in adapting electrophoretic media for optimum performance in light modulators. One important factor is minimization of haze. In this application, “haze” refers to the percentage of diffuse transmitted light (light that is scattered as it is transmitted), compared to total transmitted light. In order to create a variable transmission film (VTF) with reduced haze it is important to match the refractive index of all components present at non-planar interfaces. In capsule-based electrophoretic media such non-planar interfaces exist at the boundaries between the internal phase (the electrophoretic particles and the surrounding fluid) and the capsule walls, and at the boundaries between the capsules walls and the binder which, as described in the aforementioned E Ink and MIT patents and applications, normally surrounds the capsules and forms them into a coherent layer. In polymer-dispersed and some microcell electrophoretic media, such curved surfaces exist at the boundaries between the internal phase and the surrounding continuous phase or binder.
It has been found empirically that even small differences in refractive index at the relevant boundaries can produce major increases in haze; in practice, as shown in the Examples below, for every 0.001 difference in refractive index between an internal phase (the “refractive index of the internal phase” is of course in practice the refractive index of the fluid therein) and a surrounding capsule wall, haze increases by about 1 per cent, and approximately the same relationship holds for index mismatches between an internal phase and a binder with which it is contact. Hence, for most commercial applications, index matching to better than 0.010 in refractive index is needed. Achieving such tight refractive index matching poses considerable difficulties, because the materials normally used to form capsules walls or binders have considerably higher refractive indices than the fluids normally used in internal phases. For example, many of the aforementioned E Ink and MIT patents and applications describe encapsulated electrophoretic media having capsule walls formed from a coacervate of gelatin and acacia; this coacervate has a refractive index of about 1.540. The binders conventionally used in the polymer-dispersed electrophoretic media described in the aforementioned U.S. Pat. No. 6,866,760 have a refractive index of about 1.5220. In contrast, most of the aforementioned E Ink and MIT patents and applications describe electrophoretic media using hydrocarbon solvents having refractive indices of about 1.416, or mixtures of such hydrocarbon solvents with Halocarbon 0.8 oil, a low molecular weight polymer of chlorotrifluoroethylene, and such mixtures will have similar refractive indices to the hydrocarbon solvents themselves. The resultant refractive index mis-match of about 0.1 between the internal phase and the surrounding material is far too great to be tolerated in a light modulator.
The present invention provides electrophoretic media which have a low refractive index difference between the internal phase and the surrounding material and which are hence very suitable for use in light modulators.